Aluminium composite sheet material

ABSTRACT

Automotive body sheet in the form of an aluminium composite sheet material wherein a clad sheet is applied to at least one side of a core material, and wherein the core material is of an aluminium alloy selected from the group consisting of aa2xxx, aa5xxx and aa7xxx-series alloys, and wherein the clad sheet includes an AA6xxx-series alloy having less than 0.2 wt. % Cu or an AA5xxx-series alloy having less than 3.6 wt. % of Mg.

FIELD OF THE INVENTION

The invention relates to an automotive body sheet in the form of analuminium composite sheet material in which a clad sheet is applied toat least one side of a core material.

BACKGROUND TO THE INVENTION

Composite materials in brought terms are known from the state of theart, such as for example from international application WO-01/02165. Insuch a composite material the core material (which determines by far thelargest component of the composite material) mainly determines the bulkmechanical properties of the composite material, such as for example thestrength thereof. The clad sheet however (which, consequently, onlydetermines a small fraction of the composite material and thus will beof minor importance for the mechanical properties thereof) is in contactwith the environment surrounding the composite material and thus willdetermine the chemical activity, for example to a very large extent thecorrosion performance of the composite material.

As examples of aluminium composite materials according to the state ofthe art firstly brazing sheet having typically an aluminium alloy fromthe AA3xxx-series as core material (e.g. AA3003) and on one or bothsides thereof an aluminium alloy from the AA4xxx-series (e.g. AA4045 orAA4343) as clad sheet, as well as sheets for aeronautical use (corematerial comprising an aluminium alloy from the AA2xxx-series and cladsheet typically comprising an aluminium alloy from the AA1000-series)such as Alclad 2024-1230 may be mentioned. It is common for such knowncomposite materials that these are developed for a specific use and forfulfilling specific demands.

International patent application WO-00/26020 discloses a compositealuminium panel of two parallel sheets or plates secured to the peaksand troughs of a corrugated aluminium stiffener sheet. The corrugatedaluminium stiffener sheet is made from an AlMgMn alloy and having0.4-5.0% Zn. It is disclosed that the corrugated aluminium sheet has avery good corrosion resistance. However, in certain extreme corrosiveenvironment it may be useful to provide a cladding which furtherenhances to the corrosion resistance of the alloy product. Such acladding can be of an AA1000-type alloy, an AA6000-type alloy havingmore than 1% of alloying additions, and an AA7000-type alloy having morethan 0.8% of Zn such as AA7072.

International application WO-98/24571 discloses a multilayer metalcomposite product obtained by compound strand casting. The productcomprises a core, preferably an aluminium alloy, on at least one side ofthe core an interlayer bonded to the core and a cladding bonded to theinterlayer. Depending on the application of the product, the claddingforming the outersurface of the composite product can be a aluminiumbrazing alloy for use in brazing sheet, an AA1xxx-series alloy to obtaina mirror like surface finish, or an zinc containing aluminium alloy orzinc or a zinc alloy to improve corrosion resistance.

As will be appreciated herein below, except as otherwise indicated, allaluminium alloy designations refer to the Aluminum Associationdesignations in Aluminium Standards and Data and the RegistrationRecords, as published by the Aluminium Association in 2006.

For this invention “sheet product” refers to a rolled product form over0.15 mm through 2.5 mm in thickness with sheared, slit, or sawed edges.

For this invention “automotive body sheet” or “ABS” refers to aluminiumalloy sheet for automotive body applications, in particular exteriorpanels, interior panels and structural parts.

For any description of alloy compositions or preferred alloycompositions, all references to percentages are by weight percent unlessotherwise indicated.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improvedaluminium composite material of the type referred to above, which isdesigned for use as automotive body sheet.

This and other objects and further advantages are met or exceeded by thepresent invention concerning automotive body sheet in the form of analuminium composite sheet material wherein a clad sheet is applied to atleast one side of a core material, and wherein the core materialconsists of an aluminium alloy selected from the group consisting ofAA2xxx, AA5xxx and AA7xxx-series alloys, and wherein the clad sheetconsists of an AA6xxx-series having less than 0.2 wt. % Cu or anAA5xxx-series alloy having less than 3.6 wt. % of Mg.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides automotive body sheet in the form of analuminium composite sheet material wherein a clad sheet is applied to atleast one side of a core material, and wherein the core materialconsists of an aluminium alloy selected from the group consisting ofAA2xxx, AA5xxx and AA7xxx-series alloys, and wherein the clad sheetconsists of an AA6xxx-series having less than 0.2 wt. % Cu or anAA5xxx-series alloy having less than 3.6 wt. % of Mg.

To date panels for use in vehicles (also referred to as Automotive BodySheet or ABS) did use only a single aluminium alloy (and thus definewhat is referred to as non-clad or non-composite systems). Generally,aluminium alloys from the AA5xxx-series are used for inner panelapplications, whereas aluminium alloys from the AA6xxx-series are usedfor outer panel and structural applications. Although by varying thechemical composition of the alloys a wide variation of mechanicalproperties can be achieved, not all possibilities are applied inpractice. For applications in vehicles these possibilities are limited,among others, due to restrictions imposed by corrosion performance. Forexample, an increase of the Mg-level in AA5xxx-series aluminium alloysindeed enhances the formability as well as strength, but at the sametime the corrosion performance (especially the so-called intergranularcorrosion) is worsened. Likewise, in AA2xxx- and AA7xxx-series aluminiumalloys, an increase of respectively the Cu-level and/or Zn-level (whichwould be beneficial for the strength-performance) would adversely leadto an increased corrosion sensitivity (especially the so-called filiformcorrosion) for painted surfaces, which has restricted usage of thesecompositions in automotive body sheet applications (inner panel, outerpanel and structural applications).

Thus, although the skilled person might want to increase the Mg-, Cuand/or Zn-level in the aluminium sheet for increasing the mechanicalproperties, the subsequent reduction of the chemical properties, and thecorrosion resistance in particular, does not allow it.

According to the present invention the properties of the core sheetmaterial are disconnected from the demands imposed on the surface of thecomposite material. Thus, the core material can be chosen such as tomeet the mechanical demands as good as possible (meaning for theabove-mentioned core alloys an increase of the Mg, Cu and Zn levels),whereas the clad sheet can be chosen such as to meet the demands oninteraction with the environment. It has been found that a combinationof a core material comprising an aluminium alloy from the AA2xxx-,AA5xxx- or AA7xxx-series with a clad sheet comprising an aluminium alloyselected from the group consisting of the AA5xxx-series with less than3.6 wt. % Mg, and AA6xxx-series with less than 0.2 wt. % Cu can meetsuch demands.

Furthermore, in vehicles adhesive bonding of automotive body sheetsoccurs, regulated by specific requirements (although not as stringent asis the case in the aeronautical field). While a non-composite systemneeded an adequate pre-treatment of its surface, it appears that thecomposite sheet material according to the present invention does notneed so to obtain a sufficient durability as this is already providedfor by the choice of the clad layer.

According to an embodiment the AA5xxx-series aluminium alloy of the corehas the following chemical composition, in weight percent:

-   -   Mg 3.6-6.0, preferably 4.0-4.7    -   Mn 0.05-1.2, preferably 0.07-0.8, more preferably 0.2-0.55    -   Zn<0.9, preferably <0.45, more preferably <0.3    -   Fe<0.55, preferably <0.25    -   Si<0.45, preferably <0.2    -   Cr<0.3, preferably 0.05-0.25, for example 0.08 to 0.2    -   Zr<0.3, preferably 0.05-0.25, for example 0.07 to 0.15    -   Cu<0.5, for example <0.15 or 0.2 to 0.5    -   Ti<0.2, for example 0.01 to 0.04,    -   other elements and inevitable impurities, each element<0.05,        total<0.2, balance aluminium.

This compositional range encompasses in particular the aluminium alloysfrom the group of AA5022, AA5023, AA5082, AA5182, AA5186, AA5059,AA5083, AA5058 and AA5088.

Suitable AA2xxx-series core alloys are those having a Cu-content up to6.8 wt. %, and preferably up to 5.0 wt. %, in particular AA2219, AA2024,AA2324, AA2008, AA2010 and the AA2036-series alloy are suitable forautomotive body sheet when clad according to the present invention.

Suitable AA7xxx-series core alloys are those having a Zn-content of upto 9.7 wt. %, and preferably up to 6.5 wt. %, and a Cu-content of up to2.6 wt. %, and preferably up to 2.0 wt. %, in particular the AA7020,AA7021, AA7029, AA7050, AA7075, AA7003 and AA7004-series alloy aresuitable for automotive body sheet when clad according to the presentinvention.

These alloys are extremely well-fitted for the intended use in theautomobile field because of their specific properties, which per se arewell known to the skilled person.

For AA5xxx-series core material, this provides the advantage that thegood formability of the 5xxx-series alloy can be combined with the verygood dent resistance and filiform and intergranular corrosion resistanceof the herein defined AA6xxx-series type clad alloy.

For AA2xxx and AA7xxx core material, this provides the advantage thathigh Cu or high Zn variants like e.g. AA2219 and AA7050 can be chosen asa core for automotive body sheet, so that an overall higher strength isobtained. With respect to hemming performance and corrosion resistance,these type of alloys are generally known to be inferior to that of e.g.a AA6016-series alloy. By cladding the high Cu and/or Zn containing corematerial with an AA6xxx-series clad material having a low Cu content ofless than 0.2%, an overall high strength can be combined with a goodhemming performance and a good resistance to filiform and intergranularcorrosion, yielding a unique combination of properties. It has beenfound also that an improved hemming performance results also in animproved crash performance of the automotive body sheet according to thepresent invention. In particular AA6016-series clad alloys are preferredbecause of their very good hemming performance, corrosion resistance andtheir high dent resistance.

In another embodiment the core is clad with an aluminium alloy from theAA5xxx-series with less than 3.6 wt. % of Mg amongst others to obtain agood intergranular corrosion resistance, particularly suitable alloysare chosen from the AA5754, AA5051A or AA5018-series alloys.

The composite sheet product in accordance with the present invention maycomprise only one clad sheet applied to only one side of the corematerial. In another embodiment a clad sheet is applied to both sides ofthe core material. As a result, the composite material exhibitsexcellent balanced properties, viz. strength and formability versuscorrosion performance, dent resistance and hemming performance.

Although the dimensions of the aluminium composite material can bevaried in many ways (mostly imposed by the specific use and concurrentdemands), for use as automotive body sheet the core material has athickness in the range of about 0.5 to 2 mm, preferably about 0.7 to 1.3mm, and most preferably about 1 mm. The clad layer or clad layers areusually much thinner than the core sheet, each clad layer constitutingabout 1 to 25% of the total composite sheet thickness. A clad layer orclad sheet more typically constitutes around about 1 to 12% of the totalcomposite sheet thickness.

In a further aspect of the invention it relates to an automotive bodypanel and automotive structural parts made from the automotive bodysheet in the form of a composite sheet material according to thisinvention.

In a further aspect the invention relates to a method for producing analuminium composite material in which a clad sheet is applied to atleast one side of a core material. In accordance with the presentinvention the core material comprises an aluminium alloy from thedefined AA2xxx-, AA5xxx- or AA7xxx-series, and wherein the defined cladsheet is attached to the core material by means of roll bonding toachieve the require metallurgical bonding between the core sheet and thecladding.

Such a roll bonding process is very economic and results in a veryeffective composite sheet material presenting the desired properties.

Of course, the roll bonding process may be accompanied by severaladditional processing steps such as for example annealing fordetermining the final properties of the automotive body sheet in theform of the composite sheet material.

When carrying out such a roll-bonding process for producing the sheetmaterial according to the present invention, it is preferred that boththe core material and clad sheet experience a thickness reduction duringthe role bonding.

For example the core material initially can be an about 400 mm thickblock with at both sides a clad sheet initially about 24 mm thick. Afterroll bonding the final width of the core material was about 1 mm,whereas the final thickness of the clad layer is, for example, about 60μm.

It is noted that the initial dimensions and final dimensions of thecomposite sheet product will be determined both by the specifics of theroll-bonding process as well as the required properties of the finalsheet product.

The roll bonding process can be carried out in different manners. Forexample, it is possible that the roll-bonding process includes both hotrolling and cold rolling.

Further, the roll-bonding process may be a one-step process or amulti-step process in which during successive rolling steps the materialis gauged down. Separate rolling steps then may be separated by otherprocessing steps, for example annealing steps, heating steps, coolingsteps, etc.

In another embodiment according to this invention the defined claddingsheet is attached to the core material by means of a casting technique,for example as disclosed in EP-1638715, incorporated herein byreference.

As expressed above and set forth in the claims, the aluminium compositematerial in accordance with the present invention is for use asautomotive body sheet in vehicles. The choice of the core material fromthe AA2xxx-, AA5xxx- or AA7xxx-series leads to excellent mechanicalproperties as these are substantially determined by the core sheet, andwhich are at least comparable with standard industrial sheet alloys usedfor this application. Tests conducted on specimens of the aluminiumcomposite sheet material according to the present invention has showedfurther that there is no significant correlation between the thicknessof the clad sheet and the mechanical properties, thus confirming whathas been stated above with respect to disconnecting the bulk propertiesof the core material from the demands imposed on the clad sheet.

Further, the choice of in particular AA6016-series aluminium alloy asclad sheet enables to obtain an excellent performance of the compositematerial as regards to the environment. For example, its sensitivity forintergranular corrosion can be limited within acceptable limits, andfurther its sensitivity for filiform corrosion, especially incombination with painted surfaces, can be reduced.

The invention will now be illustrated with reference to non-limitingembodiments according to the invention.

Example 1

On an industrial scale three sheet products have been manufactured andprocessed to sheet product of 1 mm in an O temper. The three sheetproducts were a bare AA5182 alloy and an AA5182 alloy clad on both sideswith either an AA6016 alloy or AA5051A alloy. The AA5182 is usedfrequently for automotive body sheet applications. The core of the cladsheet product has the same thermal history as the bare sheet product,for example both have been subjected to a homogenisation treatment of 10hours at 500° C. The clad sheet product was made using common rollbonding practices of hot-rolling the clad product to an intermediategauge of 7.5 mm and whereby the hot-mill exit temperature was about 300°C., and then cold rolled to a final gauge of 1 mm. The clad sheet had atotal thickness of 1 mm and each clad layer had a thickness of 80 μm.

The exact alloy compositions are listed in Table 1. All products havebeen solution heat treated at 550° C. and then quenched and then after 2weeks at room temperature tested while being in the so-called O-temperfor its strength, total elongation and hemming performance. Thestrength, intergranular corrosion resistance and dent-resistance havebeen measured also simulated cycle whereby the product in the O-temperis further subjected to a 2% cold stretch following by a heat-treatmentof 20 minutes at 185° C.

Both the bare sheet product and the clad sheet products have been testedin the O temper for their hemming performance via a flat hemming test(bending the samples 180° with a bending radius of 0.0 mm as included inASTM norm E290-97A) and followed by a visually assessed. A score wasgiven according to the following rating: rating “5” represents no visualdefects, “4” mild surface roughening, “3” severe surface roughening, “2”small surface cracks, and “1” represents continuous surface cracks, andwhereby a further sub-rating of for example 3¼, 3½ and 3¾ is used.

Both the bare sheet product and clad sheet products have been assessedfor their mechanical properties according to ASTM norm EN10002 fortensile tests. The tensile properties have been determined in thereferenced O-temper and also after a simulated cycle. The intergranularcorrosion resistance (“IGC) after the simulated cycle has been measuredaccording to the European norm ASTM G67-86 and the result is expressedas weight loss expressed in mg/cm². In addition the staticdent-resistance after the simulated cycle has been measured whereby theproduct is mounted and subsequently loaded with a steel indentor havinga radius of 63.5 mm with a speed of 2 mm/min, such that from theforce-displacement curve the static dent-resistance F_(0.1mm) isdetermined as the force (in N) required to make an indention of 0.1 mmdeep. The test results are listed in Table 2.

TABLE 1 Alloy composition in wt. % of the AA6016, AA5051A, and AA5182,balance aluminium and inevitable impurities. Alloy Si Fe Cu Mn Mg Cr TiAA6016 1.0 0.23 0.15 0.07 0.60 0.03 0.02 AA5051A 0.1 0.2 0.02 0.05 1.850.15 0.02 AA5182 0.1 0.16 0.12 0.24 4.9 0.03 0.01

TABLE 2 Test results of the bare alloy sheet and the alloy sheet cladwith AA6016 and AA5051A. Bare AA5182 with AA5182 with Property andcondition AA5182 AA6016 clad AA5051A clad Yield strength (MPa) 117 121113 in O temper Total elongation (%) 28.5 27.7 27.9 in O temper Hemmingin O temper 3½ 3.0 3½ Yield strength after 154 171 144 2% + 185° C./20min IGC (mg/cm²) after 21 2 3 2% + 185° C./20 min Dent-resistanceF_(0.1 mm) (N) 165 181 161 after 2% + 185° C./20 min

From the results of Table 2 it can be seen that the clad sheet productwith the AA6016 cladding has in increased strength due to the strongercladding, whereas the formability expressed in elongation remainseffective unchanged. Together with the strength also the dent-resistancefavourably increases. Despite the use of a AA6xxx-series cladding it hasbeen found that the hemming performance of the clad sheet product is notadversely affected, while the corrosion performance is significantlyimproved. Although not mentioned in Table 2, but it has had also beenfound that the susceptibility of the clad product to roping incomparison to the bare sheet product was unchanged.

From the results of Table 2 it can also be seen that the clad sheetproduct with the AA5051A cladding has a slightly reduced strength anddent-resistance compared to the unclad sheet product, while theformability expressed in elongation remains effectively unchanged. Butfor this slight reduction in strength a very advantageous improved incorrosion performance is obtained. Furthermore can it be seen that thehemming performance is not affected by the clad sheet.

Although not mentioned in Table 2, but has had also been found that thesusceptibility of the clad product to roping in comparison to the baresheet product was unchanged.

In the present example the core alloys have been clad on both sides forpractical reasons in the production thereof, but it will be immediatelyapparent to the skilled person that the same benefits can be obtained byusing only a single clad sheet or clad layer.

This example illustrates the principle of the present invention that thecorrosion performance of an AA5000-series alloy suitable for automotivebody sheet can be improved by providing it with a suitable claddinglayer, while still benefiting from the favourable characteristics of thecore sheet such as its strength, formability, hemming performance anddent resistance.

The invention is not limited to the embodiments described before, whichmay be varied widely within the scope of the invention as defined by theappending claims.

1. Automotive body sheet in the form of an aluminium composite sheetmaterial wherein a clad sheet is applied to at least one side of a corematerial, and wherein the core material consists of an aluminium alloyselected from the group consisting of AA5xxx and AA7xxx-series alloys,and wherein the clad sheet consists of an AA6xxx-series having less than0.2 wt. % Cu or an AA5xxx-series alloy having less than 3.6 wt. % of Mg.2. Sheet material according to claim 1, wherein the core material is aAA5xxx-series alloy having the following composition, in wt. %: Mg3.6-6.0 Mn 0.05-1.2 Zn<0.9 Fe<0.55 Si<0.45 Cr<0.3 Zr<0.3 Cu<0.5 Ti<0.2,inevitable impurities, each<0.05, total<0.2, balance aluminium.
 3. Sheetmaterial according to claim 2, wherein the core material has aZn-content of <0.45%.
 4. Sheet material according to claim 2, whereinthe core material has a Mg-content of 4.0 to 4.7%.
 5. Sheet materialaccording to claim 2, wherein the core material has a Fe-content of<0.25%.
 6. Sheet material according to claim 2, wherein the corematerial has a Mn-content of 0.07 to 0.8%.
 7. Sheet material accordingto claim 1, wherein the core material comprises an aluminium alloyselected from the group consisting of AA5022, AA5023, AA5082, AA5182,AA5186, AA5059, AA5083, AA5058 and AA5088-series alloy.
 8. (canceled) 9.Sheet material according to claim 1, wherein the core material is anAA7xxx-series alloy comprising Zn at most 9.7 wt. %, and/or Cu at most2.6 wt. %.
 10. Sheet material according to claim 1, wherein the corematerial consists of an aluminium alloy selected from the groupconsisting of AA7020, AA7021, AA7029, AA7050, AA7075, AA7003, andAA7004-series alloy.
 11. Sheet material according to claim 1, whereinthe clad sheet consists of the aluminium alloy AA6016.
 12. Sheetmaterial according to claim 1, wherein the clad sheet consists of analuminium alloy selected from the group consisting of AA5754, AA5051A,and AA5018.
 13. Sheet material according to claim 1, wherein the corematerial consists of an aluminium alloy selected from the groupconsisting of AA5022, AA5023, AA5082, AA5182, AA5186, AA5059, AA5083,AA5058 and AA5088-series alloy, and the clad sheet consists of analuminium alloy selected from the group consisting of AA5754, AA5051A,and AA5018.
 14. Sheet material according to claim 1, wherein the corematerial consists of an aluminium alloy selected from the groupconsisting of AA5022, AA5023, AA5082, AA5182, AA5186, AA5059, AA5083,AA5058 and AA5088-series alloy, and the clad sheet consists of anAA6xxx-series having less than 0.2 wt. % Cu, and preferably AA6016. 15.Sheet material according to claim 1, wherein the core material has athickness in a range of 0.5 to 2 mm.
 16. Sheet material according toclaim 1, wherein the composite sheet material consists of a corematerial having a clad sheet on one side only.
 17. Sheet materialaccording to claim 1, wherein the composite sheet material consists of acore sheet having a clad sheet on both sheet surfaces.
 18. Sheetmaterial according to claim 1, wherein the one or both clad sheets havea thickness in a range of 1 to 25% of the total thickness of thecomposite sheet material.
 19. Sheet material according to claim, whereinthe one or both clad sheets have a thickness in a range of 2 to 12% ofthe total thickness of the composite sheet material.
 20. Automotive bodypanel made from the automotive body sheet according to claim
 1. 21.Sheet material according to claim 1, wherein the core materialAA5xxx-series alloy has 4.0-4.7 Mg.
 22. Sheet material according toclaim 1, wherein the core material AA5xxx-series alloy has 0.2-0.55 Mn.23. Sheet material according to claim 2, wherein the core material has aZn-content of <0.3%.
 24. Sheet material according to claim 2, whereinthe core material has a Mn-content of 0.2 to 0.55%.
 25. Sheet materialaccording to claim 1, wherein the core material is an AA7xxx-seriesalloy comprising Zn at most 6.5 wt. %, and/or Cu at most 2.0 wt. %. 26.Sheet material according to claim 1, wherein the core material has athickness in a range of 0.7 to 1.3 mm.